This invention relates generally to boron carbide-aluminum ceramic-metal composites (cermets) suitable for use as structural parts and their preparation. This invention relates particularly to cermets yielding structural parts that can withstand prolonged exposure, in air, at temperatures of 625.degree. Centigrade (.degree. C.) or above. This invention relates more particularly to such cermets that have a surface layer of aluminum oxide (Al.sub.2 O.sub.3)and their preparation.
U.S. Pat. No. 4,605,440 discloses a process for preparing boron carbide-aluminum composites. The process includes a step of heating a powdered admixture of aluminum (A1) and boron carbide (B.sub.4 C) at a temperature of 1050.degree. C. to 1200.degree. C. The process depletes of most of the Al by forming a mixture of several ceramic phases that differ from the starting materials.
U.S. Pat. No. 4,702,770 discloses a method of making a B.sub.4 C--Al composite. The method includes a preliminary step of heating B.sub.4 C powder, in the presence of free carbon, at temperatures ranging from 1800.degree. C. to 2250.degree. C. This step reduces reactivity of B.sub.4 C with molten Al. During this step, the B.sub.4 C particles from a rigid network that, after infiltration by molten Al, substantially determines mechanical properties of resulting composites.
U.S. Pat. No. 4,718,941 discloses a method of making metal-ceramic composites from ceramic precursor starting constituents. The constituents are chemically pretreated, formed into a porous precursor and then infiltrated with molten reactive metal. The chemical pretreatment alters starting constituent surface chemistry and enhances molten metal infiltration. Ceramic precursor grains, such as B.sub.4 C particles, held together by multiphase reaction products formed during infiltration constitute a rigid network that substantially determines mechanical properties of a resultant composite.
In preparing a B.sub.4 C--Al cermet via infiltration of molten Al into a porous B.sub.4 C preform, reactivity depends primarily upon reaction time. This poses a major problem because chemistry changes as a front of molten Al moves into the preform. The change in chemistry results in a cermet with large differences in microstructure. A portion of the preform that first comes into direct contact with infiltrating metal differs significantly, in terms of amount of reaction phases and reaction phase morphology, from a portion that comes into direct contact with infiltrating metal at or near completion of infiltration. These differences lead to residual stresses that promote cracking of resulting cermets.
Post-infiltration heat treatments of a cermet, typically conducted in a vacuum or in argon at temperatures exceeding that at which metal components of the cermet melt, lead to two additional problems. First, they promote reductions in free metal content and make initial differences even more pronounced. As a result, cracking increases in severity. Second, the heat treatments result in increased porosity, especially at or near external surfaces of the cermet. The porosity results because molten Al does not wet B.sub.4 C or most boron-aluminum-carbon (B--Al--C) phases at temperatures of less than 900.degree. C. As a result, surface tension forces metal toward external surfaces of the cermet, thereby creating a zone of porosity.